The present invention relates to a system for driving a non rigid exploration device into wellbores where its progression by gravity is difficult.
In a general way, the system according to the invention is suitable each time the friction forces are sufficient to prevent the progression of an exploration device along a borehole. This may happen because of a restriction of the section of a well and/or because of its substantial inclination in relation to the vertical.
The system according to the invention can for example be used for driving up to horizontal areas of a well an emission-reception device for acoustic, electric, electromagnetic signals, etc, of any type.
In the field of acoustic waves for example, the design of an emission-reception system is different depending on whether a more or less long-range exploration of the land areas crossed by a well by means of low frequency acoustic waves is favoured, in order to study the limits of a possible reservoir, or a more localized study of the formations around such a well.
It is well-known that the most interesting results, when low frequency acoustic waves are used, are obtained by substantially moving away the emitters and the receivers. This can be achieved by arranging a seismic source at the surface and by displacing a reception set in a deflected well zone at a given depth under the surface.
French Patent 2,609,105 corresponding to U.S. Pat. No. 4,945,987, describes a method for carrying out measurings in a well zone that is strongly inclined in relation to the vertical by means of a sonde for wells containing appropriate sensors and fitted with one or several retractable arms whose opening allows to press it against the walls. The sonde is fastened to the end of a tubing and linked to the latter by retractable locking means. It is taken down and pushed up to the area of action by a tubular column progressively formed by the successive interconnecting to the first one of a series of additional tubing sections. The sonde is linked with a surface installation by a multifunction cable. Interconnecting the cable with the sonde is preferably achieved when the latter has reached a certain depth. The cable, fitted with a socket connector that can be plugged in in a liquid medium, is introduced into the column through a lateral window in a special connection sub (side-entry sub). The connector is pushed until it plugs into a contact plug fastened to the locking means and linked to the sonde by a linking cable. When the sonde has been pushed up to the intervention area, the opening of the locking means which fasten it to the bottom of the column and the opening of its fastening arms are remote controlled through the cable. The sonde can then be detached from the column by moving the latter back and the waves emitted at the surface can then be received.
Emission-reception systems where the emission means are also taken down into a wellbore are well-known. The emission means and the reception means can be contained in the same well tool or in different tools hanging one under another.
A sizeable space between the emitters and the receivers can be obtained quite easily in the wells or in portions of vertical wells by lengthening the cables linking the sonde or the main tool with the satellites hanging below. A system suitable for substantially vertical wells is for example described in French Patent 2,616,230.
Nevertheless, such a device emitting and receiving acoustic waves with multiple, very spaced out sondes, becomes totally ineffective in cases where the progression by gravity cannot be achieved normally because of excessive friction forces, as it happens in well zones with a limited section or too much inclined in relation to the vertical.
French Patent EN. 89/04,554 corresponding to U.S. patent application Ser. No. 505,902, filed Apr. 6, 1990 describes a seismic prospecting method in deflected wells by means of an emission-reception set of acoustic waves displaceable in relation to the lower back end of a tubular column taken down in a well. The emission-reception set comprises a receiving sonde with retractable fastening arms arranged at the bottom of the column and linked to a moving element displaceable within the latter. It also comprises an acoustic source inserted on the column. The source can be fixed in relation to the column or displaceable in relation to the latter by means of the moving element. The wall of the column is fitted with lateral openings allowing the emission of acoustic waves towards the formations around the well. A multifunction cable fitted with a socket plug that can be plugged in in a liquid medium allows a delayed connection of the emission-reception set with a surface control and recording installation. The system is operating by fastening the sonde and by drawing it apart from the lower end of the column.
This prior system is suitable for prospecting operations utilizing sources that can be seated within the relatively narrow tubular columns which are generally used in wellbores. Sparkers can for example be utilized as sources.
Besides, the source being inserted on the tubular column, it emits its energy through slits in the wall. Part of the emitted energy tends to be transmitted along the column. Absorbing means must therefore be interposed in the portion of the tube between the source and the receiving sonde, in order to avoid direct transmissions towards the pickups.
It is also well-known that, in the field of acoustic or seismic wave prospecting, there are numerous treatment methods allowing to make the subsoil cross-sections obtained from the picked up and recorded signals more legible, by combining recordings of signals picked up in several different reception locations spaced out from one another along the well. This is not possible with the systems utilizing only one receiving sonde that are currently used, because of operating difficulties in the deflected wells.